Re: [PATCH v1] ACPI / scan: Acquire device_hotplug_lock in acpi_scan_init()

David Hildenbrand
2019-07-25T14:35:12Z

On 25.07.19 15:57, Michal Hocko wrote:
> On Thu 25-07-19 15:05:02, David Hildenbrand wrote:
>> On 25.07.19 14:56, Michal Hocko wrote:
>>> On Wed 24-07-19 16:30:17, David Hildenbrand wrote:
>>>> We end up calling __add_memory() without the device hotplug lock held.
>>>> (I used a local patch to assert in __add_memory() that the
>>>> device_hotplug_lock is held - I might upstream that as well soon)
>>>>
>>>> [ 26.771684] create_memory_block_devices+0xa4/0x140
>>>> [ 26.772952] add_memory_resource+0xde/0x200
>>>> [ 26.773987] __add_memory+0x6e/0xa0
>>>> [ 26.775161] acpi_memory_device_add+0x149/0x2b0
>>>> [ 26.776263] acpi_bus_attach+0xf1/0x1f0
>>>> [ 26.777247] acpi_bus_attach+0x66/0x1f0
>>>> [ 26.778268] acpi_bus_attach+0x66/0x1f0
>>>> [ 26.779073] acpi_bus_attach+0x66/0x1f0
>>>> [ 26.780143] acpi_bus_scan+0x3e/0x90
>>>> [ 26.780844] acpi_scan_init+0x109/0x257
>>>> [ 26.781638] acpi_init+0x2ab/0x30d
>>>> [ 26.782248] do_one_initcall+0x58/0x2cf
>>>> [ 26.783181] kernel_init_freeable+0x1bd/0x247
>>>> [ 26.784345] kernel_init+0x5/0xf1
>>>> [ 26.785314] ret_from_fork+0x3a/0x50
>>>>
>>>> So perform the locking just like in acpi_device_hotplug().
>>>
>>> While playing with the device_hotplug_lock, can we actually document
>>> what it is protecting please? I have a bad feeling that we are adding
>>> this lock just because some other code path does rather than with a good
>>> idea why it is needed. This patch just confirms that. What exactly does
>>> the lock protect from here in an early boot stage.
>>
>> We have plenty of documentation already
>>
>> mm/memory_hotplug.c
>>
>> git grep -C5 device_hotplug mm/memory_hotplug.c
>>
>> Also see
>>
>> Documentation/core-api/memory-hotplug.rst
>
> OK, fair enough. I was more pointing to a documentation right there
> where the lock is declared because that is the place where people
> usually check for documentation. The core-api documentation looks quite
> nice. And based on that doc it seems that this patch is actually not
> needed because neither the online/offline or cpu hotplug should be
> possible that early unless I am missing something.

I really prefer to stick to locking rules as outlined on the
interfaces if it doesn't hurt. Why it is not needed is not clear.

>
>> Regarding the early stage: primarily lockdep as I mentioned.
>
> Could you add a lockdep splat that would be fixed by this patch to the
> changelog for reference?
>

I have one where I enforce what's documented (but that's of course not
upstream and therefore not "real" yet)

commit 263da346cd3cf526de3f5138827fbc3520f2f8e0
Author: David Hildenbrand <david@redhat.com>
Date: Fri Jun 21 12:05:39 2019 +0200

mm/memory_hotplug: Assert that the device_hotplug_lock is held

We currently need the device_hotplug_lock(), as documented. Let's assert
that the lock is held when adding/removing/onlining/offlining memory.

Updated documentation to make this clearer.

Signed-off-by: David Hildenbrand <david@redhat.com>

That patch in return was the result of debugging a lockdep warning
we can trigger right now (and I think it's a false positive
prevented by the device_hotplug_lock - I think it is the tie breaker).
Anyhow, this patch here didn't change it.

1. Start a guest with a DIMM attached
2. Online a memory block of that DIMM
3. Unplug the DIMM

:/# [ 22.616108] Offlined Pages 32768
[ 22.631567]
[ 22.632337] ======================================================
[ 22.635104] WARNING: possible circular locking dependency detected
[ 22.637475] 5.3.0-rc1-next-20190723+ #111 Not tainted
[ 22.639314] ------------------------------------------------------
[ 22.641276] kworker/u4:0/8 is trying to acquire lock:
[ 22.642578] (____ptrval____) (kn->count#39){++++}, at: kernfs_remove_by_name_ns+0x3c/0x80
[ 22.645004]
[ 22.645004] but task is already holding lock:
[ 22.646495] (____ptrval____) (mem_sysfs_mutex){+.+.}, at: remove_memory_block_devices+0x65/0xd0
[ 22.648649]
[ 22.648649] which lock already depends on the new lock.
[ 22.648649]
[ 22.650488]
[ 22.650488] the existing dependency chain (in reverse order) is:
[ 22.651987]
[ 22.651987] -> #4 (mem_sysfs_mutex){+.+.}:
[ 22.653254] __mutex_lock+0x8d/0x930
[ 22.654079] create_memory_block_devices+0xa4/0x140
[ 22.655292] add_memory_resource+0xd6/0x200
[ 22.656252] __add_memory+0x58/0x90
[ 22.657096] acpi_memory_device_add+0x149/0x2b0
[ 22.658126] acpi_bus_attach+0xf1/0x1f0
[ 22.658899] acpi_bus_attach+0x66/0x1f0
[ 22.659698] acpi_bus_attach+0x66/0x1f0
[ 22.660482] acpi_bus_attach+0x66/0x1f0
[ 22.661265] acpi_bus_scan+0x3e/0x90
[ 22.662098] acpi_scan_init+0x104/0x24d
[ 22.662920] acpi_init+0x2ab/0x30d
[ 22.663733] do_one_initcall+0x58/0x2cf
[ 22.664727] kernel_init_freeable+0x1b8/0x242
[ 22.665780] kernel_init+0x5/0xf1
[ 22.666494] ret_from_fork+0x3a/0x50
[ 22.667271]
[ 22.667271] -> #3 (mem_hotplug_lock.rw_sem){++++}:
[ 22.668378] get_online_mems+0x39/0xc0
[ 22.669327] kmem_cache_create_usercopy+0x29/0x280
[ 22.670369] kmem_cache_create+0xd/0x10
[ 22.671412] ptlock_cache_init+0x1b/0x23
[ 22.672206] start_kernel+0x225/0x501
[ 22.672979] secondary_startup_64+0xa4/0xb0
[ 22.673887]
[ 22.673887] -> #2 (cpu_hotplug_lock.rw_sem){++++}:
[ 22.675091] cpus_read_lock+0x39/0xc0
[ 22.675962] __offline_pages+0x3e/0x7c0
[ 22.676997] memory_subsys_offline+0x3a/0x60
[ 22.678073] device_offline+0x82/0xb0
[ 22.679039] acpi_bus_offline+0xdb/0x150
[ 22.679912] acpi_device_hotplug+0x1b4/0x3a0
[ 22.680939] acpi_hotplug_work_fn+0x15/0x20
[ 22.682025] process_one_work+0x26c/0x5a0
[ 22.683019] worker_thread+0x48/0x3e0
[ 22.683942] kthread+0x103/0x140
[ 22.684855] ret_from_fork+0x3a/0x50
[ 22.685841]
[ 22.685841] -> #1 (&device->physical_node_lock){+.+.}:
[ 22.687246] __mutex_lock+0x8d/0x930
[ 22.688179] acpi_get_first_physical_node+0x18/0x60
[ 22.689699] acpi_companion_match+0x3b/0x60
[ 22.690989] acpi_device_uevent_modalias+0x9/0x20
[ 22.692626] platform_uevent+0xd/0x40
[ 22.693832] dev_uevent+0x86/0x1c0
[ 22.695133] uevent_show+0x93/0x100
[ 22.695988] dev_attr_show+0x14/0x40
[ 22.697342] sysfs_kf_seq_show+0xb2/0xf0
[ 22.698845] seq_read+0xd0/0x3f0
[ 22.700066] vfs_read+0xc0/0x170
[ 22.701168] ksys_read+0x63/0xe0
[ 22.702392] do_syscall_64+0x4b/0x1b0
[ 22.703979] entry_SYSCALL_64_after_hwframe+0x49/0xbe
[ 22.705708]
[ 22.705708] -> #0 (kn->count#39){++++}:
[ 22.707658] __lock_acquire+0xe2f/0x1a20
[ 22.708877] lock_acquire+0x95/0x190
[ 22.710299] __kernfs_remove+0x253/0x2f0
[ 22.711936] kernfs_remove_by_name_ns+0x3c/0x80
[ 22.713392] device_del+0x148/0x360
[ 22.714685] device_unregister+0x9/0x20
[ 22.716414] remove_memory_block_devices+0x90/0xd0
[ 22.718135] try_remove_memory+0xc6/0x130
[ 22.719669] __remove_memory+0x5/0xc
[ 22.721178] acpi_memory_device_remove+0x72/0xf0
[ 22.723178] acpi_bus_trim+0x50/0x90
[ 22.724537] acpi_device_hotplug+0x222/0x3a0
[ 22.726257] acpi_hotplug_work_fn+0x15/0x20
[ 22.728044] process_one_work+0x26c/0x5a0
[ 22.729825] worker_thread+0x48/0x3e0
[ 22.731128] kthread+0x103/0x140
[ 22.732137] ret_from_fork+0x3a/0x50
[ 22.733368]
[ 22.733368] other info that might help us debug this:
[ 22.733368]
[ 22.736178] Chain exists of:
[ 22.736178] kn->count#39 --> mem_hotplug_lock.rw_sem --> mem_sysfs_mutex
[ 22.736178]
[ 22.739723] Possible unsafe locking scenario:
[ 22.739723]
[ 22.741143] CPU0 CPU1
[ 22.741788] ---- ----
[ 22.742653] lock(mem_sysfs_mutex);
[ 22.743990] lock(mem_hotplug_lock.rw_sem);
[ 22.746069] lock(mem_sysfs_mutex);
[ 22.747207] lock(kn->count#39);
[ 22.748132]
[ 22.748132] *** DEADLOCK ***
[ 22.748132]
[ 22.749182] 7 locks held by kworker/u4:0/8:
[ 22.750684] #0: (____ptrval____) ((wq_completion)kacpi_hotplug){+.+.}, at: process_one_work+0x1e9/0x5a0
[ 22.753966] #1: (____ptrval____) ((work_completion)(&hpw->work)){+.+.}, at: process_one_work+0x1e9/0x5a0
[ 22.756429] #2: (____ptrval____) (device_hotplug_lock){+.+.}, at: acpi_device_hotplug+0x2d/0x3a0
[ 22.758292] #3: (____ptrval____) (acpi_scan_lock){+.+.}, at: acpi_device_hotplug+0x3b/0x3a0
[ 22.759836] #4: (____ptrval____) (cpu_hotplug_lock.rw_sem){++++}, at: try_remove_memory+0x3b/0x130
[ 22.761463] #5: (____ptrval____) (mem_hotplug_lock.rw_sem){++++}, at: percpu_down_write+0x1b/0xf2
[ 22.763812] #6: (____ptrval____) (mem_sysfs_mutex){+.+.}, at: remove_memory_block_devices+0x65/0xd0

--

Thanks,

David / dhildenb